An apparatus and method for determining the condition of an electricity-producing cell such as a fuel cell is disclosed. A signal is injected into an electricity-producing cell, a voltage and/or current response is measured, and the impedance response of the electricity-producing cell is calculated using the injected signal and the response. The injected signal is a broadband signal that includes a plurality of superimposed waveforms at different frequency set points across a frequency range. The distribution of the waveform frequency set points is linear at either or both of a lower portion and an upper portion of the frequency range, and is logarithmic at a mid-range of the frequency range. The response at each of the frequency set points are simultaneously obtained and the impedance response across the frequency range is calculated and used to determine a condition of the electricity-producing cell.